Actions geometrical data and design situations

C8.3.1 Actions

Besides listing relevant actions, 2.4.2 also requires the use of Cases A, B and C and the partial load factors to be applied. These are to be applied to retaining structures, with the special note in 2.4.2(17) about the application of Case B. The clauses of Section 8 refer directly to 'design values' rather than characteristic values. These design values should generally be derived from characteristic values in accordance with 2.4.2.

C8.3.1.1 Weight of backfill material

C8.3.1.2 Surcharges

Unlike BS 8002, Eurocode 7 does not impose a minimum design surcharge. The authors recommend that designers make their own assessment of the characteristic surcharge. In some cases, especially where the retaining walls are small, the surcharge may be appreciably less than the rather severe value required by BS 8002.

C8.3.1.3 Weight of water C8.3.1.4 Wave forces C8.3.1.5 Supportingforces

For the purpose of checking the stability of a retaining wall, the force in a dead-man anchor will generally be a result of the wall-ground interaction and so it is not an action, in that calculation. However, a prestressed anchor could, in principle, have a prestress force which is chosen by the designer, together with an additional component which results from the wall-ground interaction. In this case, for the purpose of the stability check, the prestress is an action, and the additional component is a reaction, not an action. In practice, many prestressing systems are fairly extensible, so the 'reaction' component is small compared with the prestress.

It was noted in C2.4.2 that the same force may be a reaction in some calculations and an action in others. For prestressed anchors, for example, the 'reaction' to wall-ground interaction, could be regarded as an action for the purpose of the structural design of the wall. In this case it might be either permanent or, if dependent on other variable loading such as traffic or tides, it could be variable.

EC2, Table 2.2 gives values for partial factors on prestress actions in prestressed concrete, but separate values for ground anchors are not provided in the Eurocode system.

The sequence of evaluation of the various forces involved in design of ground anchors is discussed in C8.8.2.

C8.3.1.6 Collisionforces

C8.3.1.7 Temperature effects C8.3.1.7(2)

The effect of temperature on prop loads has been considered in the recent CIRIA report 'Prop loads: guidance on design' (Twine and Roscoe (1997)).

C8.3.2 Geometrical data

In most cases, small variations in geometrical data are considered to be accommodated by the safety elements, mainly partial factors, included in the calculations (C2.4.5). However, because the design of retaining walls is extremely sensitive to ground levels and water levels, special requirements are included in this subclause. (In 6.5.4, a similar exception was made for loads with large eccentricities, for which a direct allowance for construction tolerance of a spread foundation was required.)

This paragraph requires that the ground level of passive soil should be assumed to be slightly lower than the lowest the designer expects to occur. This is not intended to give the designer or constructor permission to over-excavate in front of the wall. Rather, it is an allowance for the unforeseen activities of nature or humans who have no technical appreciation of the stability requirements of the wall.

The requirements of EC7 are similar to those of BS 8002, but not identical. BS 8002 requires a minimum allowance of 0.5 m, whereas Eurocode 7 sets 0.5 m as a maximum. This paragraph is an application rule, and so It is permissible to use alternative rules differentfrom the application rules given in this Eurocode, provided it is shown that the alternative rules accord with the relevant principles (1.3 (5)P). In 8.3.2.1, (2) is an application rule of the principle (1)P, so some discretion is left with the designer. An alternative wording of this paragraph has been proposed by Krebs Ovesen and Simpson as follows (the main changes being shown in bold type):

In ultimate limit state calculations in which the stability of a retaining wall depends on the passive resistance ofthe ground infront ofthe structure, the ground level ofthe passive soil should be lowered below the nominal, expected level by an amount Aa. The value of Aa should be selected taking account of the degree of control to be exerted on site over the level of the surface. For situations with a normal degree of control, the following should be applied:

a for a cantilever wall, Aa should equal [10%] of its height, limited to a maximum of[0.5] m.

b for a supported wall, Aa should equal [10%] of the height beneath the lowest suprport, limited to a maximum of [0.5] m.

Smaller values of Aa, including zero, may be used where the surface level is to be controlled reliably throughout the period in which it is operational.

Larger values of Aa should be used where the surface level is particularly uncertain.

It is recommended that this ground level reduction should only be disregarded with great caution, particularly where embedded walls rely heavily on relatively short penetrations into the restraining soil. Using the EC7 boxed values for soil strength factors, Simpson (1994) showed that for small penetrations the effect of this allowance was to give an equivalent factor of safety on passive pressure exceeding 2, whereas it drops to about 1.5 without this allowance. This is illustrated in Figure C8.1. Hence, if the ground level allowance is disregarded, the values of partial factors to be used in the calculations may need amendment. Figure C8.1 Overall factor of safety on Kp derived from boxed values with overdig allowance The original purpose of this (after Simpson (1994)) allowance was described by Simpson

(1992). Embedded walls which have only small penetrations into soils with high shear strength (or high angles of shearing resistance) are very sensitive to any reduction in the ground level in front of the wall. Figure C8.2 compares two excavations in dense sand or gravel (0k'= 40°)- In excavation (a), the wall would be at failure (for (j)^) with a penetration of 1.25 m, but with a penetration of 2.0 m it would have Figure C8.2 Effect of overdig on factor of safety of a wall with small penetration into dense reasonable factors of safety of 1.2 on granular material (after Simpson (1992)) tan<|>' or 2.0 on passive pressure (note that EC7 requires 1.25 on tan(j)'). However, excavation (b) shows that if the excavation is taken only 0.5 m deeper by any accidental or natural process, the wall again reaches limiting stability. The dramatic effect of this small 'overdig' has led the drafters ofboth EC7 and BS 8002 to require that a direct allowance is made for it.

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  • Zack
    What is an overdig allowance in design?
    2 years ago

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